Phosphate systems are used to pre-treat steel parts and assemblies of steel parts before they are painted. For example, phosphate systems are used in automotive assembly to pre-treat a vehicle sub-assembly commonly known as a body-in-white before it is painted. Such phosphate systems typically have a series of sprays and full immersion dips that the body-in-white passes through. A typical phosphate treatment for a body-in-white involves cleaning, rinsing, surface activation, phosphating, rinsing, a neutralizing rinse, drying and the application of supplemental coatings.
FIG. 1 is a simplified diagram of an eight stage prior art phosphate system 100 of the type used in the automotive industry. There is an entry transfer station 102 at an entry side 104 of the phosphate system 100 and an exit transfer station 106 at an exit side 108 of phosphate system 100. Phosphate system 100 includes a plurality of spray stages 110, 112, 114, 116 and a plurality of full immersion dip stages 118, 120, 122, 124, 126. Each full immersion dip stage 118, 120, 122, 124 includes a full immersion dip tank 127 containing a solution that is recirculated by a pump (not shown) through a filter (not shown). Full immersion dip stage 118 is a cleaning stage with its full immersion dip tank 127 containing a cleaning solution 119, full immersion dip stage 120 is a conditioner stage with its full immersion dip tank 127 containing a conditioner solution 121, full immersion dip stage 122 is a zinc phosphate stage with its full immersion dip tank 127 containing a zinc phosphate solution 123, full immersion dip stage 124 is a city water rinse stage with its full immersion dip tank 127 containing a rinse solution that is city water, and full immersion dip stage 126 is a deionized water rinse and conditioner stage with its full immersion dip stage containing a deionized water and conditioner solution 129. Spray stages 110, 112, 114, 116 are water spray rinse stages.
A metal structure 128, for example a body-in-white 130, is conveyed to entry transfer station 102 where it is placed on a skid 132 and the skid hung on a plurality of hangers 134, typically one hanger 134 at each corner of the skid 132. The hangers are attached to an overhead conveyer (not shown) that lowers and raises the hangers to lower and raise the skid as applicable as it progresses through phosphate system to dip the body-in-white in the full immersion dip tanks. After the skid progresses through the phosphate system 100, the body-in-white is removed from the skid at exit transfer station 106. While only one skid 132 is shown in FIG. 1, it should be understood that there are a number of skids 132 hung on respective hangers 134 that in sequence carry respective metal structures 128 through phosphate system 100 with each skid 132 carried by respective hangers 134 following a preceding skid 132 carried by respective hangers 134.
With reference to FIG. 2, an example of hanger 134 is shown. Hanger 134 has a shaft 200 that attaches at an upper end 202 of shaft 200 to an overhead conveyor (not shown). A lower end 204 of shaft 200 attaches to the skid 132. In an example, hanger 134 has a hook 206 that hooks onto the skid 132. In an aspect, the hook 206 is attached to lower end 204 of hanger 134 and in another aspect, lower end 204 is formed with a hook shape to provide hook 206. A typical hanger 134 as shown in the example of FIG. 2 is J-shaped and is commonly known as a J-hook. It should be understood in other aspects, hanger 134 has shapes other than a J-shape, for example shaft 200 having a C-shaped hook at its lower end 204 or lower end 204 being formed in a C-shape.
During welding of metal structure 128 such as body-in-white 130, weld balls 136 are produced and loosely adhere to surfaces of the metal structure, typically in weld seams and in other crevices. While the weld balls 136 can typically be cleaned off the surfaces of the exterior of the metal structure before the metal structure enters phosphate system 100, it is more difficult to clean the weld balls 136 that are in the interior of the metal structure 128, such as in the interior of body-in-white 130. As the metal structure 128 progresses through phosphate system 100, weld balls 136 fall off. Weld balls 136 disposed in the solutions can be deposited on surfaces of the metal structure 128, typically on surfaces of metal structures subsequently passing through the full immersion dip tanks. If weld balls 136 remain on surfaces of the metal structure 128 when it is painted, they cause blemishes in the painted surface of the metal structure 128.
The solutions used in the full immersion dip tanks are recirculated and pass through filters which filter debris that may be suspended in the solutions. While these filters remove some of the weld balls 136, they do not remove all of them. They typically reduce the concentration of weld balls 136 making it less likely that a weld ball will be deposited on a surface of the metal structure as it is being dipped in a full immersion tank. However, with use, the filters become increasingly clogged and are less effective in removing weld balls 136. The filters are thus periodically cleaned and also periodically replaced. As can be appreciated, as time elapses from the time a filter is cleaned or replaced, the more likely it will be that the fewer weld balls 136 will be removed. The weld balls 136 are also the main source of debris that fills up the filters, reducing the cycle that the filters can be used without cleaning or replacing.